1. Field of the Invention
This invention relates to an apparatus and method for dry-forming a fibrous structure, and more specifically to an apparatus and method employing a unique seal arrangement to prevent the loss of fibers during high speed formation of the fibrous structure.
2. Description of the Prior Art
Low basis weight nonwoven fibrous webs in the basis weight range of from about 1 oz/yd.sup.2 to about 6 oz/yd.sup.2 are becoming extremely popular. Some of these webs are formed predominately of short cellulosic fibers less than one-fourth inch in length, and include a minor proportion, by weight, of longer reinforcing fibers having an average length greater than one-fourth inch. These webs can be used by themselves, or in conjunction with other materials as substitutes for conventional textile fabrics in articles such as disposable diapers, sanitary napkins, industrial and household wipers, cosmetic pads and the like.
U.S. Pat. No. 3,862,472, issued to Norton et al, and assigned to Scott Paper Company, discloses a highly desirable method for dry-forming a low basis weight nonwoven fibrous web which includes short and long fibers which are blended together. In accordance with that method loosely compacted batts of the short and long fibers are directed into a fiber blending device in which the fibers are individualized from their respective batts and blended together to form a fibrous feed mat. The feed mat is directed into a web forming device in which the fibers are again separated into their individual fiber components and redeposited on a forming surface in the form of a fibrous web having a basis weight in the range of from about 1 oz/yd.sup.2 to about 6 oz/yd.sup.2. The fibrous feed mat has a basis weight at least 3 times the average basis weight of the low basis weight nonwoven fibrous web to be formed, and in all cases, has a basis weight greater than 12 oz/yd.sup.2.
A need continuously exists for upgrading processes and apparatus for dry-forming low basis weight nonwoven fibrous webs, and in particular, to achieve faster web forming speeds. By increasing formation speeds dry-forming processes become more competitive with conventional papermaking processes which employ water as the carrying medium for the fibers.
In the Norton et al process the blending operation, in which the individual batts of long and short fibers are separated into their individual fiber components and blended together, has been a significant speed limiting factor in the formation of low basis weight fibrous webs. Specifically, devices of the general type employing a main drum and satellite worker rolls are usable to accomplish the blending operation at low speeds. However, at high-production speeds these prior art devices do not reliably process fibrous material in which the predominate fiber component, by weight, is less than one-fourth inch in length, such as wood pulp fibers and cotton linters. The reason why these prior art devices are unable to handle short fibers at high production speeds is clearly set forth in copending application Ser. No. 715,165, filed on even date herewith, and covering APPARATUS AND METHOD FOR FORMING FIBROUS STRUCTURES invented by Joel P. Gotchel and Henry J. Norton (hereinafter referred to as the "Gotchel-Norton application"). The Gotchel-Norton application is incorporated by reference into the instant application.
The apparatus disclosed in the Gotchel-Norton application is a carding machine adapted for use in the high speed processing of fibrous material in which the predominate fiber component, by weight, is less than one-fourth inch in length. That apparatus includes a unique arrangement of baffles and air control means for permitting a main drum of the apparatus to be rotated with a peripheral speed of over 10,000 feet/minute to blend long and short fibers together in the formation of a fibrous feed mat. The baffles and air control means direct air and fibers along the periphery of the main drum during high speed operation of the device. It was found that the absence of a seal arrangement adjacent the bottom of the main drum resulted in a loss of a considerable percentage (i.e. 10-20%) of the textile-length fibers being processed in the apparatus. These long fibers were dragged by the high speed rotation of the main drum along a lower casing member of the apparatus, and escaped from the apparatus adjacent the inlet section for the fibrous feed. As a result of the loss of long fibers, the percentage of long fibers included in the fibrous feed had to be greater than that which was to be included in the formed mat. This obviously increased the raw material costs associated with the mat forming operation.
U.S. Pat. No. 3,051,998, issued to Rust, Jr. et al, discloses an air seal for preventing fibers from draping over the edge of a formation duct in a dry-forming apparatus. Rust et al indicates that, in the absence of an air seal, an area of stagnant air exists adjacent the edge of the formation duct closest to the toothed cylinder to permit fibers to drape about that edge. In the Rust et al apparatus an air stream is directed through an air-seal counter to the air stream induced by rotation of the toothed cylinder. This displaces the location of stagnant air to a point remote from edge of the formation duct to prevent fibers from contacting and hanging-up on the edge of said duct.
Applicants have found that an air seal, by itself, is not totally effective in preventing an air stream induced by the rotation of a toothed main drum from conveying textile-length fibers around the main drum, under the usual stationary cover, and back to the feed area at which the fibers escape from the apparatus. Rust et al indicate that an acceptable pressure in a nozzle plenum is from 5 inches to 7 inches of water. In the devices and methods of interest to applicants such pressures are not suitable for preventing the main drum from conveying, to the feed area, air which is trapped between the teeth of said main drum. Directing such a flow of air into the feed area disturbs the integrity of the fibrous feed, and causes said feed to move in an intermittent fashion into engagement with the toothed periphery of the main drum. This adversely affects the uniformity of the machine-direction basis weight in the fibrous structure formed from the fibrous feed.
U.S. Pat. No. 3,949,035, issued to Dunning et al, discloses an air-seal arrangement similar to that employed in the Rust et al apparatus. In fact, the flow of air through the air seal in the Dunning et al apparatus is set to prevent the draping of fibers over the upper edge of the formation duct wall, in the same manner as disclosed in the Rust et al patent. The air seal employed in the Dunning et al device is believed to be deficient for the same reasons discussed above in connection with the Rust et al device.